1. Field of the Invention
The present invention relates to an engine oil composition excellent in high-temperature detergency and low fuel consumption characteristic.
2. Description of Related Art
Engine oil is conventionally used, in a circulating manner, in an internal combustion engine for lubricating various slide portions such as sliding surfaces between a piston ring and a cylinder liner, bearings of a crankshaft and a connecting rod, valve systems including cams and valve lifters, drive devices such as an oil pump and a distributor.
In such an internal combustion engine, since the temperature between the piston ring and the cylinder liner is very high, a soot-like substance called “deposit” is generated, and the generated deposit adheres around a piston-ring groove. The deposit is considered to be derived from engine oil and fuel under the influence of heat and oxygen. The performance of the internal combustion engine (especially low fuel consumption characteristic) will be seriously affected if the amount of the deposit increases. Thus there has been a desire for an engine oil which produces minimum deposit, namely, an engine oil which has so called high-temperature detergency.
In order to prevent the deposit from being generated and maintain low fuel consumption characteristic for long period of time, an engine oil composition obtained by adding a molybdenum-contained friction modifier and a boron compound into a base oil has been proposed (refer to, for example, Document 1: Japanese Patent Laid-Open Publication No. Hei08-283762).
Another proposal which can prevent the deposit from being generated at high temperature is an engine oil composition having kinematic viscosity of 2-13cSt at 100° C. and containing 1 wt. % (based on the total amount of the composition) or more base oil that has boiling point of 480° C. or higher measured by gas chromatograph distillation (refer to, for example, Document 2: Japanese Patent Laid-Open Publication No. Hei09-328694).
However, moisture will generally be generated inside an engine along with combustion of fuel (gasoline, diesel oil, etc.), therefore in the case where the engine oil composition described in Document 1 is used, hydrolysis reaction will happen due to the contact between the boron compound and the moisture, so as to form a gel substance. Accordingly, it will be not enough to prevent the deposit from generating.
Further, a lower engine oil viscosity will generally lead to a lower stirring resistance, namely lead to a low fuel consumption characteristic. Since the engine oil composition described in Document 2 contains heavy component having boiling point of 480° C. or higher, it will have high viscosity at low temperature, and thereby the low fuel consumption characteristic at cold start will be impaired.